[llvm] [IR][DAG] Add support for `nneg` flag with `uitofp` (PR #86141)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

[MachineInstr](https://llvm.org/doxygen/classllvm_1_1MachineInstr.html) and `MachineInstr::copyFlagsFromInstruction` will eventually need to support the nneg flag.

https://github.com/llvm/llvm-project/pull/86141